Family
A teen's environment at home plays a large role in their mental and emotional development. If parents place a lot of pressure on teens, forcing them to partake in extracurricular activities, leaving little time for fun or friends, it may contribute to depression. A harmful home environment, such as abusive parents or inadequate support from parents may also contribute to depression. The religious and political views of a family are another possible source of stress in teens.
School
Problems with school can contribute greatly to depression among teenagers. There is often a lot of pressure on teens to perform well in classes from parents and teachers, so a failure to meet those expectations can cause emotional strain. Also, teens are often aware of the grades of other students---struggling with material that other students find easy can create feelings of inadequacy, which may contribute to depression.
Relationships
Teen relationships, both romantic and friendships, are a potential source cause of a depressed mental state. Teens are usually inexperienced in relationships, and may not have the emotional maturity to appropriately handle complex problems. Teens are prone to deep infatuation and feelings of love, so a breakup with a boyfriend or girlfriend can cause tremendous emotional stress, which may lead to depression.
Social Status
Teenagers face complex social environments, where the desire to fit in to a certain group can be of overriding importance. If a teen is rejected by a group that they want to fit into, or prohibited from being a part of a certain social group by a parent, it can contribute to depression. Pressure to wear certain clothes and maintain a certain style can also increase stress among teens, especially if that style is difficult to maintain due to their family's socio-economic status.
Bullying
Bullying is another factor that can cause depression among teens. Being picked on by a bully creates feelings of weakness and inadequacy and can make a teen afraid to go to school. Fear of being bullied can become thought consuming, causing the teen to lose focus on classes and activities that they enjoy. Since boys are often taught to be strong and not to express emotion, boys that are bullied may keep their problem a secret, allowing emotional stress to build, which can feed depression.